You'll want to copy the /languages/en-GB/en-GB.com_jfbconnect.ini file to your locale's language directory and rename the file's locale prefix as well. Once there, you can edit the strings as necessary. The specific string for the button is "LOGIN USING FACEBOOK".
Hope this helps, but if you need more info, just let us know.
Finally, I'll be moving this topic to the Support section after I post this, so find this post there if you have any other questions. Thanks!